How can I make a camera the active one?
There are a few ways to set the active camera.

Key Binding: select it and press Ctrl+ Numpad 0.

From the menu:3D Viewport Header -&gt; View -&gt; Cameras -&gt; Set active object as camera.

Scene property:
Scene tab in the Properties Editor and pick a camera object from the list:





Outliner (V2.90+):

You can click the green camera icon beside the camera's name in the outliner to toggle it as active for the scene.

Animation:
To change the active camera during an animation you can use Markers.

Add a marker where you want to change the active camera (M in the Timeline, or Timeline header &gt; Add marker will add a marker to the current frame):



Select the object which you want to set as the new active camera and the marker which you want to bind it to, then press CtrlB in the Timeline (or Timeline header &gt; View &gt; Bind camera to markers):


Note that markers behave like keyframes, so you will need at least two markers with cameras bound to them to have any camera switching.
To illustrate this, here is an example with Camera.001 bound to the marker on frame 1, and Camera bound to the marker on frame 5:


Note: If setting the active camera is only effecting a single view, you may have Scene-lock disabled. In the 3D View header (Right hand side of layers buttons), there is a lock icon. This should normally be pushed in, unless you explicitly want to set a camera &amp; layers only to a single view.